Clean up an Office file before you send it
A .docx, an .xlsx and a .pptx are not files: they are archives full of parts. Inside travel your name, how long you worked on it, the tracked changes nobody accepted, the comments and the hidden sheets. Here you see everything that is in there, remove the personal properties and take home the same document, clean.
What an Office file carries with it
Change a .docx extension to .zip and open it: inside there is no document, there are dozens of parts. One of them, docProps, is written by nobody: Office fills it in on its own with the name of whoever created the file, the name of the last person who saved it, how many minutes it was kept open, the company name typed when the computer was set up and the number of times it was saved. None of this shows when you open the document, which is why it sits there for years, from file to file, because almost every document is born from another document.
What I remove, and what I leave to you
The properties are mine to remove: they are self-contained parts of the archive, they are rewritten without touching the document, and every other part is copied over exactly as it is, still compressed, so the text, the pictures and the formulas you download are bit for bit the ones you started with. Tracked changes, comments, hidden sheets and speaker notes I do not remove: they live inside the body of the document, and removing them means deleting the references scattered through the text as well. A single reference left behind greets whoever opens the file with «Word found unreadable content», which for a tool about privacy is the worst possible outcome. So I count them for you, name them and write down where they come off in the real program.
Tracked changes and comments are the real trap
The classic disaster is not the author's name: it is the contract sent to the client with the tracked changes still in it, where the figure you had written before correcting it can be read, or the comment «we drop this, they will never notice» left in a corner. Whoever gets the file often does not even have to look for them: Word shows them by itself, if their markup view is on. Hiding them on screen does not take them out of the file; accepting or rejecting them does.
Excel: hidden sheets and the pivot cache
Excel holds something almost nobody knows about: a pivot table carries a copy of the source data inside the file. If you delete the sheet with your buying prices but leave the pivot, the prices are still there, and whoever gets the file only needs a double click on a total to bring them all back in a fresh sheet. The same goes for hidden sheets, which come back with two clicks, and for «very hidden» ones, which the menu will not show but whose data sits in the file exactly like everything else.
Where this stops
The file never leaves your browser: it is opened here, stitched back together here and downloaded from here. The old formats do not open (.doc, .xls, .ppt), being not archives but an entirely different format, and neither do password-protected files, which are encrypted: those have to be opened and saved again in Office first. If all you want is to READ what is inside, there are Open a Word file for the text and Excel (XLSX) to CSV for the cells. For photographs the hidden data is a different matter and there is Remove photo metadata, which strips location and camera model.